1. Field of the Invention
The present invention generally relates to couplers for coupling optical radiation into and out of an optical fiber.
2. Description of Related Art
Optical fibers have by far the greatest transmission bandwidth of any conventional transmission medium, and therefore optical fibers provide an excellent transmission medium. An optical fiber is a thin filament of drawn or extruded glass or plastic having a central core and a surrounding cladding of lower index material to promote internal reflection. Optical radiation (i.e. light) is coupled (i.e. launched) into the end face of an optical fiber by focusing the light onto the core. For effective coupling, light must be directed within a cone of acceptance angle and inside the core of an optical fiber; however, any light incident upon the surrounding cladding or outside of the acceptance angle will not be effectively coupled into the optical fiber.
It is a difficult task to couple light into the central core of an optical fiber due to its small size and acceptance angle, particularly if the optical fiber is a single mode optical fiber. A typical single mode fiber has a core diameter of only 10 microns and an acceptance angle of only 10°. Single-mode fibers, which are designed to transmit only single-mode optical radiation, are widely utilized for telecommunications applications. Multimode optical fibers have a larger cross-section and a larger acceptance angle than single-mode fibers. For example, a typical multimode fiber has a core diameter of 50 microns and an acceptance angle of 23°. Because any optical radiation outside the core or acceptance angle will not be effectively coupled into the optical fiber, it is important to precisely align the core with an external source of optical radiation.
One conventional practice for making a fiber-pigtailed transmitter is to assemble an edge-emitting laser diode, an electronics circuit, a focusing lens, and a length of optical fiber and then manually align each individual transmitter. To align the transmitter, the diode is turned on and the optical fiber is manually adjusted until the coupled light inside the fiber reaches a predetermined level. Then, the optical fiber is permanently affixed by procedures such as UV-setting epoxy or laser welding. This manual assembly procedure is time consuming, labor intensive, and expensive. Up to 80% of the manufacturing cost of a fiber-pigtailed module can be due to the fiber alignment step. The high cost of aligning optical fiber presents a large technological barrier to cost reduction and widespread deployment of optical fiber modules.
One single-mode fiber has a cylindrical glass core of about 10 microns in diameter surrounded by a glass cladding with a circular outer diameter of about 125 microns. In some connections, slight variations in dimensions can drastically affect coupling efficiency, and therefore some optical fiber manufacturers carefully control the fiber's tolerances. For example, in a splice connection between two optical fibers, a large loss in the transmitted signal can occur if the two inner cores fail to align precisely with each other. For example, if the cores of two 10 micron single-mode fibers are offset by only 1 micron, the loss of transmitted power through a splice is about 5%. Therefore, to reduce coupling losses, manufacturers maintain cladding diameter tolerances within the micron to sub-micron range. For example, Corning Inc. specifies the tolerance of its optical fibers as 125±1 micron.
In order to provide passive alignment of optical fibers, various alignment techniques have been reported based on precisely etched holes on a wafer. For example, in Matsuda et al. “A Surface-Emitting Laser Array with Backside Guiding Holes for Passive Alignment to Parallel Optical Fibers”, IEEE Photonics Technology Letters, Vol. 8 No. 4, (1996) pp. 494-495, a research group at Matsushita in Japan performed an experiment in which a shallow guiding hole on the backside of a back-emitting vertical cavity surface emitting laser (VCSEL) wafer is etched to a depth of 10 to 15 microns and a diameter of 130 microns. A multi-mode fiber stem 125 microns in diameter is inserted into the guiding hole with a drop of epoxy for passive alignment to the VCSEL. This group reported an average 35% coupling efficiency at 980 nanometers. The large core diameter of multi-mode fibers (e.g. 50 microns) allows this approach to be suitable for coupling light into multi-mode fibers; however the lack of a light-focusing mechanism prevents use of this method with single-mode fibers.
U.S. Pat. No. 5,346,583 to Basavanhally discloses a substrate having at least one lens formed on a first surface. An optical fiber guide is etched on a second surface of the same substrate, opposite the first surface. The optical fiber guide is used to mount an optical fiber on the second surface such that the central axis of the optical fiber is substantially coincident with the central axis of the lens, thereby giving the desired alignment. Fused silica and silicon are two common substrate materials. If the substrate material is fused silica (or glass), the fiber guide etch rate is very slow (typically 0.3 micron per minute or less) and as a result it is impossible to obtain fiber guides of sufficient etch depth, which is necessary to obtain precise angular alignment to single mode fibers. According to the method described in the patent, etching is to stop before it reaches the final surface where the lens resides. At the bottom of the etched fiber guide, the surface is typically neither smooth nor flat, which could causescattering and reflection loss if the refractive index of the substrate material is different than that of the optical fiber core (approximately 1.5).
U.S. Pat. No. 5,195,150 to Stegmueller et al. discloses an optoelectronic device that includes a substrate that has a recess for receiving a plano-convex lens and a recess on the other surface of the substrate aligned with the lens to receive an end of an optical fiber. The device disclosed by Stegmueller is subject to the same problems as the device disclosed in the Basavanhally patent.